The present invention relates to an apparatus and a method for controlling an electric power steering system used in vehicles such as automobiles.
An electric power steering system uses an electric motor to assist movement of a steering wheel. The electric motor generates assist force to reduce force for manipulating a steering wheel, or steering force. The electric motor is controlled by a control value outputted by a control apparatus. The control value is calculated by the control apparatus based on the deviation between a control target value and a driving current that actually flows through the electric motor. An assist current value, which is a control target value related to the assist force, is calculated by the control apparatus based on the steering torque of the steering wheel. Hereinafter, this control, which is executed based on the assist current value, will be referred to as assist control.
A typical prior art control apparatus discontinues the assist control when there is an abnormality in a power supply circuit connected to the torque sensor or when the level of the detection signal is out of a normal range and such abnormality continues over a predetermined determination time. Upon stopping the assist control, the control apparatus turns off a power supply relay for supplying electric power to the motor and a relay provided in a drive circuit of the motor.
The control apparatus includes a detecting circuit that detects the motor driving current and the motor terminal voltages. Based on the detection result, an abnormality of the motor and an abnormality of a circuit that drives the motor are detected. When such abnormal condition continues over a predetermined time, the control apparatus discontinues the assist control and turns off the relays.
Conventionally, the determination time is relatively short so that, if there is an abnormality as described above, the relays are turned off at the same time as the assist control is stopped.
When detecting abnormalities using a short determination time, a detected abnormality may be dissolved after the determination time (for example, when the abnormality is caused by noises and temporally). In this case, an abnormality is erroneously detected.
Accordingly, it is an objective of the present invention to provide a control apparatus of an electric power steering, which control apparatus prevents abnormalities in circuits from being erroneously detected.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a control apparatus of an electric power steering system is provided. The electric power steering system is coupled to a steering wheel. The electric power steering system includes a motor and a circuit related to control of the motor. The motor generates assist force for reducing steering force of the steering wheel. The apparatus has calculating means, controlling means, determining means, and canceling means. Calculating means calculates an assist current value that corresponds to the assist force based on steering torque of the steering wheel. Controlling means executes assist control of the motor based on the assist current value such that the motor generates the assist force. Determining means determines an abnormality in the circuit. Suppressing means suppresses the assist control when an abnormality in the circuit is determined. If the determination of an abnormality has continued longer than a predetermined time since the suppression of the assist control, the suppressing means opens relay means that permits electric power to be supplied to the motor. When it is determined that the circuit returns to normal with the assist control suppressed, the canceling means cancels the suppression of the assist control.
The present invention also provides a control method of an electric power steering system coupled to a steering wheel. The electric power steering system includes a motor and a circuit related to control of the motor. The motor generates assist force for reducing steering force of the steering wheel. The method includes calculating an assist current value that corresponds to the assist force based on steering torque of the steering wheel; executing assist control of the motor based on the assist current value such that the motor generates the assist force; determining an abnormality in the circuit; suppressing the assist control when an abnormality in the circuit is determined, wherein, if the determination of an abnormality has continued longer than a predetermined time since the suppression of the assist control, a relay that permits electric power to be supplied to the motor is opened; and canceling the suppression of the assist control when it is determined that the circuit returns to normal with the assist control suppressed.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.